6 files declare their use of EntityContentBase in Zircon Profile 8

Book.php in core/modules/book/src/Plugin/migrate/destination/Book.php

Contains \Drupal\book\Plugin\migrate\destination\Book.

EntityComment.php in core/modules/comment/src/Plugin/migrate/destination/EntityComment.php

Contains \Drupal\comment\Plugin\migrate\destination\EntityComment.

EntityContentBaseTest.php in core/modules/migrate/tests/src/Unit/Plugin/migrate/destination/EntityContentBaseTest.php

Contains \Drupal\Tests\migrate\Unit\Plugin\migrate\destination\EntityContentBaseTest

EntityFile.php in core/modules/file/src/Plugin/migrate/destination/EntityFile.php

Contains \Drupal\file\Plugin\migrate\destination\EntityFile.

EntityTaxonomyTerm.php in core/modules/taxonomy/src/Plugin/migrate/destination/EntityTaxonomyTerm.php

Contains \Drupal\taxonomy\Plugin\migrate\destination\EntityTaxonomyTerm.

EntityUser.php in core/modules/user/src/Plugin/migrate/destination/EntityUser.php

Contains \Drupal\user\Plugin\migrate\destination\EntityUser.